The apparatus described herein relates generally to metrology systems for wind turbines. More specifically, the apparatus relates to a metrology system located outward from the nosecone of a wind turbine.
Recently, wind turbines have received increased attention as environmentally safe and relatively inexpensive alternative energy sources. Wind turbines do not emit greenhouse gases (GHGs), and therefore, do not contribute to global warming. Wind turbines also allow a country to become more energy independent by the domestic production of electrical energy. With the growing interest in wind generated electricity, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Wind is usually considered to be a form of solar energy caused by uneven heating of the atmosphere by the sun, irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetation. The terms wind energy or wind power, describe the process by which the wind is used to rotate a shaft and subsequently generate mechanical power or electricity.
Typically, wind turbines are used to convert the kinetic energy in the wind into mechanical power. This mechanical power may be used for specific tasks (such as grinding grain or pumping water) or a generator may convert this mechanical power (i.e., the rotation of a shaft) into electricity. A wind turbine usually includes an aerodynamic mechanism (e.g., blades or rotor) for converting the movement of air into a mechanical motion (e.g., rotation), which is then converted with a generator into electrical power.
Power output of a wind turbine generator generally increases with wind speed until a rated power output is reached. Thereafter, the power output is usually maintained constant at the rated value even with an increase in wind speed. This is generally achieved by regulating the pitching action of the blades in response to an increase in wind speed. The wind turbine could also be turned away from the wind (i.e., changing the yaw direction). With increases in wind speed beyond the rated power output, the blades generally are pitched toward feather (i.e., twisted to be more closely aligned with the direction of the wind), thereby controlling the angular speed of the rotor. As a result, generator speed, and consequently, generator output may be maintained relatively constant with increasing wind velocities.
The direction of the wind is also used to correct the yaw direction of the wind turbine. Ideally, the wind turbine should be facing into the wind, the rotor face perpendicular to the direction of wind for maximum power output. The nacelle (and blades) can rotate in a horizontal plane on top of the tower via one or more yaw drives. The yaw direction refers the horizontal direction in which the wind turbine is facing.
Typically, wind-measuring devices (e.g., anemometers, wind vanes, etc.) are mounted on the nacelle and behind the blades or rotor. The disadvantage to having the wind measuring devices mounted downwind and behind the blades/rotor, is that the blades affect the wind speed and direction. As the wind turbine rotor spins it produces various fluid phenomena such as turbulence, vortices, fluid flow shedding, etc. These rotor induced fluid phenomena produce a systematic error in a nacelle mounted wind vane (typical on commercial wind turbines) such that the turbine is not optimally aligned with the oncoming wind. The result can be less than optimum energy production, less than optimum startup operation, storm shutdown operation, and increases in mechanical stress. The same fluid phenomena effects that disturb the wind vane sensor can also effect the wind speed anemometer such that it produces erroneous signals that in turn cause significant errors in the assessment of wind turbine performance.
In case of sudden gusts, wind speed may increase drastically in a relatively small interval of time. Maintaining the power output of the wind turbine generator constant during such sudden gusts calls for relatively rapid changes of the pitch angle of the blades. However, there is typically a time lag between the occurrence of a gust and the actual pitching of the blades based upon dynamics of the pitch control actuator and the inertia of the mechanical components. As a result, generator speed, and hence power, may increase considerably during such gusts, and may exceed the maximum prescribed power output level (also known as overspeed limit) causing the generator to trip, and in certain cases, the wind turbine to shut down. The overspeed limit is generally a protective function for the particular wind turbine generator and is based upon fatigue considerations of the mechanical components, such as the tower, drive train, and so forth. Moreover, sudden gusts may also significantly increase tower fore-aft and side-to-side bending moments due to increase in the effect of wind shear.
Accordingly, there exists a need for an improved system to monitor and measure wind speed and wind direction, which may be used to control the various operating characteristics (e.g., yaw direction and pitching of the blades) of a wind turbine.